Kilowatt-hour (kWh)

One kilowatt of power sustained for one hour, and the unit in which British electricity has been bought and sold since the earliest public supply.

The kilowatt-hour is the most commercially significant energy unit in British life, since it is the basis on which every domestic electricity and gas bill is calculated. One kilowatt-hour will run a washing machine cycle, power a laptop for roughly ten hours, or heat a room with a fan heater for about two. Gas is metered by volume and then converted, first to therms and then to kilowatt-hours, so that both fuels can be compared on a single scale.